One of the largest deficiencies of known fluid recovery systems is their inability to maintain an appropriate amount of tension across their belt drives under changing conditions. Fluid recovery systems are often subjected to changing torque loads and operating speeds. By way of example, the well torque exerted on a fluid recovery system is generally higher during the upstroke (i.e., when fluid is being lifted) and lower during the downstroke (i.e., when no fluid is being lifted). Also, as the fluid levels within a well change, it is often desirable to change the pumping speed of the system to maximize recovery efficiency.
Ideally, a fluid recovery system would be able to dynamically change the amount of tension across its belt drives in response to the changing conditions described above. That is, tension should generally increase when torque demand decreases (i.e., when slack tends to develop between the belt drive components) and should decrease when torque demand increases (i.e., when the belt is pulled too tightly). If too much belt slack develops, the belt drive will slip and result in “burn out.” If the belt is stretched too tightly it will break. Problematically, known recovery systems cannot deal with these problems because their belt drive components are held at a fixed distance from one another. As a result, the tension between those components is also held fixed, irrespective of the torque load exerted upon the belt extending therebetween.
Notwithstanding the above, known fluid recovery systems are subject to other additional limitations. Namely, a number of design features found in known recovery systems leave much to be desired. These design features limit system productivity and increase operating costs, particularly where those systems are used to recover fluid while operating at low strokes per minute (SPM).
By way of example, known recovery systems are limited in efficiency because they utilize too many structural bearings. The number of structural bearings in a system is inversely related to mechanical advantage (as each bearing introduces parasitic loads in the form, e.g., friction, heat, etc.). As a natural result of these inefficiencies, known recovery units need intensive maintenance, requiring relubrication every 3 to 6 months. Also, such systems must run above a threshold stroke per minute rate (SPM) to maintain lubrication between working components. As such, these systems cannot be used for low SPM applications.
Known recovery systems are also overly expensive to build and operate as they incorporate a relatively large gearbox, which is required to handle the full torque capacity rating of the recovery unit. Larger gearboxes cost significantly more to manufacture than smaller gearboxes. Some known systems utilize a belt drive between the motor and gearbox, where the gearbox output drives the crank arm. However, such an arrangement requires that the gearbox have a torque capacity at least as large as that of the recovery unit. Other known systems utilize two belt drives but do not achieve additional speed reduction between those drives (e.g., where a flywheel is used between the first and second drive). However, such a combination is particularly undesirable for low SPM operations because of the high rotational inertia effect of the flywheel.
Moreover, the majority of known recovery units typically have symmetrical operating geometries, meaning that it takes about 180 degrees of crankshaft rotation for the recovery unit to move the polished rod up or down in the well. This is often a necessary feature where the unit is operated at a relatively high SPM rate, typically 5 SPM to 15 SPM. However, these high SPM systems waste much energy when a low volume well is being pumped.
Known recovery units tend to suffer from a high cyclic load factor (CLF), which is defined as the quotient of the root-mean-square and average motor current. The CLF can be interpreted as a measure of electrical heating and cyclic loading observed during operation. The symmetrical operating geometry and relatively high speeds associated with known recovery systems (as mentioned above) largely contribute to the high CLF of known systems.
Other systems known in the art operate at a mechanical disadvantage as they utilize either of 1) a dual pitman arm arrangement, or 2) a single pitman arm arrangement that is not counterbalanced. As a result, these systems suffer from side loading on system bearings and/or a propensity to rotate about an axis drawn between the center of the wrist pin and tail bearing. This can cause unusual wear and early failures. Known systems are at a further disadvantage because they require manual adjustments in order to maintain tension across their drive belts. That is, the distance between drive components must be manually adjusted to maintain tension on belts extending therebetween. This is cumbersome and expensive in terms of manual labor and system downtime.